GREENVILLE, S.C. — A few minutes after the Republican presidential debate concluded Saturday night, a gaggle of perplexed reporters surrounded Donald Trump's campaign manager in the spin room and took turns asking essentially the same question:

What just happened to your candidate?

For two hours, the billionaire frontrunner had let the beta male of the 2016 field, Jeb Bush, needle and bait him into a series of flustered outbursts.

It began when Bush called Trump's plan to combat ISIS "ludicrous," and the billionaire's response was interrupted by loud boos from the audience in the debate hall. Trump tried dismissing the hecklers as "lobbyists" and "Jeb's special interest," but the jeers didn't let up, and as the the debate wore on, the hostile crowd reaction seemed to take a toll on the longtime showman.

Trump, his face reddening, erupted at even the faintest prompt or mildest pushback. When a moderator pressed him on a question about Social Security, he grew increasingly defensive and disproportionately upset and by the end of the exchange he was shouting his answer.

When Ted Cruz went after him for his past positions on social issues, Trump repeatedly demanded of his opponent, "Why do you lie? Why do you lie?" until finally Cruz deadpanned, "Donald, adults learn not to interrupt people."

But it was Bush who seemed to most consistently get to Trump, and the dynamic between the two ultimately produced the only exchange of the night that might actually move votes.

When Trump was asked about comments he made years ago suggesting George W. Bush should be impeached over the Iraq War, it set off a sharp back-and-forth between him and Jeb. Amid boos from the blatantly pro-Bush audience, Trump briefly slid into a rant that made him sound like a left-wing anti-war protester circa 2004.

"You do whatever you want," Trump barked at the crowd as they booed. "You call it whatever you want. I will tell you. They lied. They said there were weapons of mass destruction and there were none. And they knew there were none. There were no weapons of mass destruction."

"We got $19 trillion in debt and the politicians don't fix it, then they put up these ads that are disingenuous, and I think it bothers Mr. Trump," Lewandowski said, not bothering to dispute the premise. "[Bush] has a super PAC that's raised over $100 million that's attacking Mr. Trump all the time, and then Jeb goes out and tries to be the happy warrior when he's got this super PAC doing all this dirty work for him."

But Bush is doing so poorly in the polls, why not go after—

"Of course he is!" Lewandowski interrupted. "You know why he's doing so poorly? Because his message is terrible. People don't want another Bush in this country.

Why does Trump dislike Bush so much?

"I don't know if it's a personal dislike..." Lewandowski started, but when the assembled reporters erupted in incredulous laughter, he recalibrated.

"Look, look, Jeb has spent $100 million in this race attacking Mr. Trump ... and I think he takes that personally."

But Bush's super PAC attacks other candidates too — why does Trump take it so personally?

"He doesn't take it personally," Lewandowski said.

And then, "No — he takes it personally because it distorts his record."

If, by some electoral miracle, South Carolina ends up being the place where Jeb turns his campaign around and ultimately overtakes The Donald, much-derided Right to Rise PAC supporting Bush will no doubt take its share of credit. After a months-long assault on Marco Rubio, the well-funded PAC has recently begun mixing in some anti-Trump material — and according to the billionaire's campaign manager, the ads are getting in his head.

Still, as Lewandowski pointed out Saturday night, the reporters peppering him with questions about Trump's unimpressive performance have been wrong before.

"The pundit class has said that every time that Trump has said something that it's going to be the end of his campaign," he said. "We've seen it I don't know 10 or 12 times. You guys have all said that. .. Then the race is over and we go up in the polls."

McKay Coppins is a senior writer for the BuzzFeed News politics team, and the author of The Wilderness, about the battle over the future of the Republican Party.